In the conventional fourdrinier papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper "stock") is fed onto the top of the upper run of an endless belt of woven wire and/or synthetic material that travels between two or more rollers. The belt, often referred to as a "forming fabric", provides a papermaking surface on the upper surface of its upper run which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium, thereby forming a wet paper web. The aqueous medium drains through mesh openings of the forming fabric, known as drainage holes, by gravity alone or with assistance from one or more suction boxes located on the lower surface (i.e., the "machine side") of the upper run of the fabric.
After leaving the forming section, the paper web is transferred to a press section of the paper machine, in which it is passed through the nips of one or more pairs of pressure rollers covered with another fabric, typically referred to as a "press felt." Pressure from the rollers removes additional moisture from the web; the moisture removal is often enhanced by the presence of a "batt" layer on the press felt. The paper is then conveyed to a drier section for further moisture removal. After drying, the paper is ready for secondary processing and packaging.
Typically, papermakers' fabrics are manufactured as endless belts by one of two basic weaving techniques. In the first of these techniques, fabrics are flat woven by a flat weaving process, with their ends being joined to form an endless belt by any one of a number of well-known joining methods, such as dismantling and reweaving the ends together (commonly known as splicing), or sewing a pin-seamable flap on each end or a special foldback, then reweaving these into pin-seamable loops. In a flat woven papermakers' fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross machine direction. In the second technique, fabrics are woven directly in the form of a continuous belt with an endless weaving process. In the endless weaving process, the warp yarns extend in the cross machine direction and the filling yarns extend in the machine direction. As used herein, the terms "machine direction" (MD) and "cross machine direction" (CMD) refer, respectively, to a direction aligned with the direction of travel of the papermakers' fabric on the papermaking machine, and a direction parallel to the fabric surface and traverse to the direction of travel. Both weaving methods described hereinabove are well known in the art, and the term "endless belt" as used herein refers to belts made by either method.
Effective sheet and fiber support and an absence of wire marking are important considerations in selecting papermaking fabrics, especially in the forming section of the papermaking machine, where the wet web is initially formed. Wire marking, which is the tendency of the paper to exhibit marks where it was supported by the yarns or wires of a papermaking fabric, is the result of individual cellulosic fibers being oriented within the paper web such that their ends reside within gaps between the individual threads or yarns of the forming fabric. Wire marking can be particularly problematic in the formation of fine paper grades, as it can affect a host of paper properties, such as sheet mark, porosity, see-through, and pin holing. This problem is generally addressed by providing a water permeable fabric structure with a substantially coplanar fine mesh that allows paper fibers to bridge adjacent yarns of the fabric rather than penetrating the gaps between yarns. As used herein, "coplanar" means that the upper extremities of the yarns defining the paper-forming surface are at substantially the same elevation, such that a substantially "planar" surface is present. Accordingly, fine paper grades intended for use in carbonizing, cigarettes, electrical condensers, quality printing, and like grades of fine paper, have typically heretofore been formed on very fine woven or wire mesh forming fabrics.
Unfortunately, such finely woven forming fabrics often are delicate and may lack dimensional stability in either or both of the machine and cross machine directions (particularly during operation), leading to a short service life for the fabric. In addition, a fine weave may adversely affect drainage properties of the fabric, thereby rendering it less suitable for use as a forming fabric.
To combat these problems associated with fine weaves, multi-layer forming fabrics have been developed with fine mesh yarns on the paper forming surface to facilitate paper formation and larger yarns on the machine contact side to provide strength and longevity. As examples, U.S. Pat. No. 4,709,732 discloses a dual layer forming fabric for use in a papermaking process, and U.S. Pat. No. 4,605,585 teaches a two-ply forming fabric with a twill or satin weave pattern.
Although double-layer fabrics have proven to be effective forming fabrics for many applications, they can be expensive to manufacture. Also, different paper varieties are generally produced on different types of fabrics. For example, a high grade paper, such as that used in magazines and printers, is typically produced on a considerably different fabric than tissue paper, which has significantly more lenient surface standards. Accordingly, fabric designers are constantly searching for new designs that provide an appropriate balance of performance characteristics and cost.
One example of a double-layer fabric which is suitable for forming tissue paper is disclosed in U.S. Pat. No. 5,025,839 to Wright. This fabric employs MD yarns that are interwoven with the machine side CMD yarns in an "under 1/over 1/under 1/over 5" pattern, and with the CMD yarns of the paper side of the fabric in an "over 1/under 2/over 1/ under 12" repeating pattern, with the MD yarns interlacing with the machine side CMD yarns in the "under 12" sections. The result of this specific pattern is that the MD yarns take a "zig-zag" configuration on the machine side of the fabric that reportedly improves drainage.
Unfortunately, this fabric has proven to be prone to "twinning" of its paper side CMD yarns in the "under 2" positions of the pattern (the positions between the locations where the MD yarns pass over the paper side CMD yarns to form paper side "knuckles"). Twinning is the tendency for adjacent paper side CMD yarns to reside near one another rather than being spaced apart a uniform distance. This is caused by tension in the machine direction yarns due to the "under 1/over 1/under 1" portion of the machine side pattern (a tension-inducing configuration). This tension forces the "under 2" paper side CMD yarns together in a "twinned" configuration. Twinning can result in uneven drainage through the paper side layer due to the disparity in drainage hole size, the result of which can be inconsistent paper surface qualities.